This invention relates to a device for measuring the position of a rotary magnetic head in a video tape recorder adapted to record and reproduce video signals.
Conventionally, in a high grade video tape recorder, magnetic head wear due to slide contact of the magnetic tape is corrected by periodically inspecting the video tape recorder.
As the magnetic head is worn, there is a corresponding change in its electromagnetic conversion characteristic. The change is corrected by readjusting the recording current and the reproducing equalizer circuit's characteristic. When it is determined that the magnetic head is worn too much to perform recording and reproducing operations, the magnetic head is replaced together with the rotary drum.
However, this method is disadvantageous in that whenever the inspection time comes, it is necessary to stop the use of the video tape recorder for inspection, even if the inspection shows that adjustment was unnecessary in fact.
In order to overcome this difficulty, a magnetic recording and reproducing device has been proposed in the art in which the amount of wear of the magnetic head is continuously measured with optical means and the recording and reproducing systems are continually adjusted according to the result of such measurement. An example of such a device is disclosed in Japanese Patent Application No. 64165/1989 owned by the assignee of the present invention.
As shown in FIG. 1 in this prior art magnetic head position measuring device 1 the output light beam LA1 of a light emitting element 2 (hereinafter referred to as "an irradiating light beam LA1", when applicable) is applied through a condenser lens LEN1 to a magnetic head 4 on a rotary drum 3 at a predetermined angle and reflected from the magnetic head 4. The light beam LA2 thus reflected (hereinafter referred to as "a reflected light beam LA2", when applicable) is applied through another condenser lens LEN2 to a position detecting element 6. Position detecting element 6 outputs a position detection signal S1 corresponding to the incident position of the reflected light beam LA2. This signal S1 is then applied to an amount-of-wear detecting circuit 7. The latter detecting circuit detects the amount of protrusion of the magnetic head beyond the peripheral surface of the rotary drum 3, i.e., determines the amount of wear of the magnetic head 4.
The condenser lens LEN1 on the irradiation side is positioned at a point along the distance between the light emitting element 2 and the magnetic head 4 is where the ratio L.sub.1 :L.sub.2 will substantially concentrate the irradiating light beam LA1 at a point M.sub.1 on the magnetic head 4.
Similarly, the condenser lens LEN2 on the reflection side is positioned at the point where the distance between the magnetic head 4 and the position detecting element 6 is divided in the same ratio of L.sub.1 :L.sub.2. Thus, the image of the point M.sub.1 on the magnetic head 4 is formed at a point N.sub.1 on the position detecting element 6.
When the magnetic head 4 is worn, as indicated by the broken line in FIG. 1, the incident position of the irradiating light beam LA1 is shifted from the point M.sub.1 to a point M.sub.2 on the prolongation of the irradiating light beam LA1. The light beam LA2 reflected from the point M.sub.2 through the center O of the condenser lens LEN2 impinges at a point N.sub.2 on the position detecting element. That is, the incident position of the reflected light beam is shifted from the point N.sub.1 to the point N.sub.2.
The amount of shift P.sub.2 (=N.sub.1 -N.sub.2) of the incident position of the reflected light beam LA2, due to the wear of the magnetic head 4, can be represented by the following equation (1), because the triangles .DELTA.OM.sub.1 M.sub.2 and .DELTA.ON.sub.1 N.sub.2, formed by the wear P.sub.1 (=M.sub.1 -M.sub.2) and the optical path difference: EQU P.sub.2 =(L.sub.1 /L.sub.2).times.P.sub.1 ( 1)
In the above-described conventional magnetic head position measuring device 1, the amount of wear P.sub.1 of the magnetic head 4 is very small, for example, a maximum of 0.05 mm. Hence, in order to increase the detection accuracy of the amount of wear P.sub.1, it is necessary to increase the ratio L.sub.1 /L.sub.2 in Equation (1), i.e., the distance L.sub.1. This means that it is necessary to increase the distance between the condenser lens LEN2 and the position detecting element 6 rendering the device unavoidably bulky.
Furthermore, in this prior art position measuring device 1, the lens system magnifies the amount of wear P.sub.1 of the magnetic head 4 only several times (about twice in practice). Therefore, the detection is relatively low in accuracy.
In addition, with this conventional position measuring device, during installation of the rotary drum 3 of magnetic head 4, a reference value for measuring the amount of wear P.sub.1, namely, a magnetic head protrusion position reference value, must be stored in a memory provided, for instance, in the amount-of-wear detecting device 7. It also is necessary to maintain the reference value in the memory.